Method of producing microporous film

ABSTRACT

This microporous film has an approximately uniform configuration in which the intervals between slender fibrils extend approximately two-dimensionally which are connected to each other between adjacent unstretched planar flat portions to define fine through-pores, and the distribution of pore size is sharp and the porosity is high. Accordingly, the microporous film excels in water filtration rate and selective separating capability and is suitable for use as filter membrane, separator membrane, gas exchange membrane, and a battery separator.

This is a division, of application Ser. No. 404,382 filed Sep. 8, 1989.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a polyolefin microporous film having amultiplicity of fine through-pores formed by a stretching method, abattery separator employing an improved porous polypropylene film, and amethod of producing them.

Microporous film of the type which is composed of film made of polymericmaterial and a multiplicity of fine through-holes formed therein areused for various applications in the fields of, for example, filtermembrane or separator membrane for use in air purification, watertreatment or the like, a separator for use in a battery or inelectrolysis, gas exchange membrane or separator membrane for use inartificial lungs or in plasma separation, and filter membrane orseparator membrane for use in sterilization and refinement of variouskinds of enzyme for production of hard or soft drinks such as beer orsake on draft or fresh juice.

A known method of producing a microporous film having a multiplicity offine through-pores comprises the steps of dispersing, for example, aneasily soluble substance in a polymeric material, forming the polymericmaterial mixed with this substance into film, and eliminating the easilysoluble substance by dissolving it with a solvent, thereby forming amultiplicity of fine pores in the film.

In recent years, another method of producing a porous body has beengenerally accepted. This method utilizes a method which comprises thesteps of forming thermoplastic crystalline polymer material into a film,then subjecting the film to heat treatment, and subsequently formingpores in the film by a stretching step.

As is known, various kinds of polymeric material can be employed as athermoplastic crystalline polymer material suitable for theabove-described purposes. Among others, a polyolefin, particularlypolypropylene, is inexpensive and excels in strength and chemicalresistance and is therefore regarded as an excellent polymeric materialsuitable for use in producing a microporous film.

A method of producing a microporous film by stretching a crystallinepolyolefin film and forming pores in the film is disclosed in, forexample, U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,801,404;3,801,692; 3,843,761 and 4,138,459. In these method, a film having poresconnected to each other therein and having an average pore size of1,000-2,000Å can be obtained. Similar methods of producing a microporousfilm are disclosed in Japanese Patent Publication Nos. 46-40119, 50-2176and 55-32531. The gist of these prior arts resides in a method whichcomprises the steps of subjecting an unstretched film to heat treatment,stretching the film at a temperature close to room temperature or atemperature in the range of between a temperature not lower than thesecond order transition temperature of the resin (for example, atemperature not lower than -40° C. in the case of polypropylene) and atemperature not higher than the temperature at which the partial meltingof the crystalline phase begins to occur. Thereby generating pores andforming a porous body, then stretching at a temperature in the range ofbetween a temperature higher than the temperature at which the partialmelting of the crystalline phase begins to occur and a temperature nothigher than the crystalline melting point, and again conducting heattreatment to thermally fix the pores thus formed.

However, in the conventionally known microporous film produced by thestretching process, the pores are defined by relatively thick portionsof nonuniform sizes and configurations which form meshes of varioussizes and meandering shapes and fine fibrils which run in these meshesin the direction approximately parallel to the direction in which thefilm is stretched. Accordingly, the configurations and sizes of thepores are nonuniform and the porosity is low. In addition, since themeandering portions form continuous closed circuits of various sizes,the longitudinal lengths of the pores which extend through the film fromone to the other surface are excessively long. Accordingly, if themicroporous film obtained by the prior art is used for plasmaseparation, the plasma filtration rate is low and the selectiveseparating capability is inferior.

A nonwoven fabric of synthetic resin or a nonwoven fabric of glass fiberis generally employed as a battery separator. In recent years,electronic devices have been made portable or even smaller in size,weight or thickness, and various kinds of card-like electronic deviceshave been being developed. In such a situation, in the field ofbatteries for use as the electrical power sources of such electronicdevices, a demand has arisen for an increase in the energy density and areduction in the thickness of the battery. One method for meeting thedemand is to improve a separator which is one element of a battery. Morespecifically, a method for using a porous film as a battery separator isproposed in order to answer the demand for an improvement in batteryperformance and a reduction in the thickness of the battery based on anincrease in the amount of active material enclosed or a reduction ininternal resistance. For example, Japanese Patent Laid-Open Nos.62-222562 and 63-126159 disclose a method of preparing a separator byimpregnating a porous polypropylene film with a liquid electrolyte or asemi-solid electrolyte.

The porosity of porous polypropylene film which has heretofore been usedis at most 45% or thereabouts. As described above, typical methods ofproducing conventional porous polypropylene film are disclosed inJapanese Patent Publication Nos. 46-40119, 50-2176 and 55-32531. Theporous polypropylene films obtained in these methods, however, present anumber of problems. For example, since the porosity is low, electrolytecannot be sufficiently retained and an area which allows ionicconduction is limited. In addition, since the overall path lengths ofthe pores from one surface of film to opposite side of film areexcessively long and the substantial distance between the electrodes islarge, the internal resistance increases. If a separator made from sucha film is used in a battery, the performance of the battery will be low.

If one intends to increase the aforesaid porosity with the prior artmethod of producing a porous polypropylene film, porosity will notincrease beyond some limiting value being less than about 55%, andnonuniform pores will be formed in the film. If such a film is used as aseparator, fine powder in an active material of either electrode maypass through the separator and contact the other electrode, thusresulting in a deterioration in the battery performance.

OBJECTS AND SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide apolypropylene microporous film which is capable of solving theabove-described disadvantages and a method of producing thepolypropylene microporous film.

It is another object of the present invention to provide a batteryseparator which excels in characteristics such as ionic conductivity anda lithium battery employing such a separator.

To achieve the above objects, the present inventors conducted intensiveresearches and succeeded in forming a microporous film having aremarkably unique structure, that is, a microporous film having a sharpdistribution of pore sizes, high porosity, and fine through-pores eachof which extends approximately two-dimensionally perpendicularly to thedirection of film stretching, by stretching an unstretched polyolefinfilm which has a specific crystallinity, small-angle X-ray scatteringdiffraction images consisting of two-point figures, a specific magnitudeof diffraction angle at the degree of diffraction which is equivalent to1/2 of the maximum degree of diffraction of each of the figures(hereinafter referred to as a "half-value width of a diffractiondegree"), and a specific long period.

More specifically, in accordance with one aspect of the presentinvention, there is provided a microporous film having a multiplicity offine through-pores obtained by stretching a polyolefin film, which filmcomprises groups of unstretched planar flat portions which run atapproximately constant intervals in the direction perpendicular to thedirection in which the film is stretched and which are formedapproximately parallel to a cross section perpendicular to the directionin which the film is stretched; and groups of relatively slender fibrilswhich run in the gap between adjacent planar flat portions atapproximately constant intervals and approximately in parallel with thedirection in which the film is stretched, both groups being combined todefine a multiplicity of fine pores each of which has an approximatelyuniform configuration in which the intervals between the slender fibrilsconnecting adjacent planar flat portions extend approximatelytwo-dimensionally.

In accordance with another aspect of the present invention, there isprovided a separator for a battery which is made from a microporous filmwhose polyolefin film is a porous polypropylene film in which a majorityof pores are through-pores and have approximately uniform pore sizes,the porosity of the film being not lower than 45%. There is alsoprovided a lithium battery employing this battery separator.

In accordance with still another aspect of the present invention, thereare provided several methods of producing the microporous film describedabove. The first method comprises the steps of preparing an unstretchedpolyolefin film, such as an unstretched polypropylene film, whosecrystallinity is 50-90%, whose diffraction images obtained by smallangle X-ray scattering are two-point figures each having anapproximately circular form, the half-value width of the maximum degreeof diffraction of each of the figures being not greater than 15', thelong period being not smaller than 120Å, and stretching the unstretchedfilm in a medium selected from the group consisting of nitrogen, oxygen,argon, carbon monoxide, methane and ethane at a stretching temperaturewhich is not higher than -70° C. and which is in the low-temperaturerange from the solidifying point of the medium to a temperature which is50° C. higher than the boiling point of the medium. The second methodcomprises the steps of preparing the above-described unstretched film,and stretching the unstretched film at a strain rate corresponding to astrain rate of less than 10%/min. in the lowest temperature within atemperature range which is 10°-60° C. lower than the melting point ofthe polyolefin employed, without any pre-stretching step at roomtemperature. In the third method, an unstretched polypropylene film issubjected to a stretching process based on the second method at astretching temperature which is in the high-temperature range of 110° to155° C.

In accordance with a further aspect of the present invention, there isprovided a method of producing a separator for use in a battery. Thismethod is substantially identical to any of the above-described methodsof producing microporous film except that it is not necessary to specifythe kind and the characteristics of unstretched polypropylene film to beemployed.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description of thepreferred embodiments thereof, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an scanning electron micrograph illustrating the configurationof fine fibrils which form a portion of the surface of a microporousfilm according to the present invention; and

FIG. 2 is an scanning electron micrograph illustrating the surface of amicroporous film and the configuration of fine fibrils on a crosssection of the same.

DETAILED DESCRIPTION OF THE INVENTION

In a microporous polyolefin film obtained by a conventionally knownmethod, for example, a film obtained by the method disclosed in JapanesePatent Publication No. 46-40119, relatively thick portions are presentin a meandering state and form closed circuits of various sizes whichmay be regarded as cells, and fine fibrils are connected to each otherin each of the closed circuits to define a multiplicity of pores.

The present inventors carefully examined various forming conditions anddeveloped a film in which these relatively thick portions were formedsolely in the direction approximately perpendicular to the direction inwhich the film was designed to be stretched, that is to say, a specialmicroporous film in which relatively thick portions of an unstretchedplanar flat shape had a multiplicity of fine pores formed in thedirection in which the film was stretched.

In other words, it may be said that the microporous film of the presentinvention has an approximately uniform configuration in which the gaps,which constitute the pores, between the fine fibrils connected betweenadjacent unstretched planar flat portions extend approximatelytwo-dimensionally.

The phrase "to extend approximately two-dimensionally" means that thepores in the microporous film of the present invention extend from oneto the other surface of the film approximately rectilinearly or in aslightly meandering state in the rightward and leftward directionsbetween two adjacent unstretched planar flat portions within the film.This configuration differs from the conventionally known configurationin which the pores in a microporous film extend from one to the othersurface of the film while meandering therein in all directions in aconfused way.

That is to say, the film provided in accordance with the presentinvention is a microporous film which overcomes the disadvantages of thefilm provided by the conventionally known method. Such a microporousfilm has pores of uniform configuration and size, as well as increasedporosity with the strength maintained, and the longitudinal size of eachpore is reduced.

In the present invention, groups of unstretched planar flat portions donot form closed circuits over the length of three time or more,preferably five times or more, more preferably ten times or more, theaverage length (d) of the fine fibrils. The average length (d) of thefine fibrils is given by selecting an arbitrary fibril which connectsarbitrary adjacent planar flat portions of the above-described ones andobtaining the average length of twenty fibrils which include thearbitrarily selected fibril and fibrils disposed therearound.

In the present invention, the thickness (Δd) planar flat portion whichis formed perpendicularly to the direction in which the film isstretched is preferably in the range of 0.1 to 40 μm, and it isdesirable that the following relationship hold between the thickness(Δd) of the flat portion and the thickness (Δl) of the fine fibril:

    3Δl≦Δd≦400Δl

If Δd is smaller than 3 Δl, the strength is insufficient, while if Δd islarger than 400 Δl, the porosity declines and the filtration ratedecreases. If the microporous film according to the present invention isto be employed as a plasma separator membrane, it is preferable that thed/l ratio of the average length (d) of the fibrils to the averageinterval (l) between the fibrils be in the range of 2 to 60 with l being0.02-2 μm.

As for the density of the fine fibrils described above, if it is assumedthat, on either of the surfaces of the microporous film of the presentinvention, a length equal to the average length (d) of the fibrils ismeasured from one starting point selected in the cross-section of theaforesaid planar flat portion which extends parallel to the direction inwhich the film is stretched, it is preferable that three to thirty finefibrils be present within the area defined by the length.

As for the physical properties of the film according to the presentinvention, the porosity is 30-85%, preferably 50-85%, the average poresize 0.02-2 μm, preferably 0.04-1.5 μm, the ratio D_(A) /D_(M) of anaverage pore size D_(A)) to a maximum pore size (D_(M)) 0.5-1.0,preferably 0.6-1.0, the tensile elastic modulus measured in thestretching direction 5,000 kg/cm² or more, preferably 6,000 kg/cm² ormore.

If the polyolefin microporous film of the present invention having theabove-described features is used as a filter membrane, no deteriorationin the filtration rate or the separating capability occurs over a longperiod of time. In addition, such a film has an interesting feature inthat if it is used for plasma separation, it does not cause anyhemolyzation at a filtration pressure below about 150 mmHg. If the filmis used as a separator in a battery, the movement of ions is facilitatedand various kinds of electrolyte can be used in accordance with the kindand configuration of the battery. It is also possible to cut theseparator into various configurations.

An example of the construction of a lithium battery employing a batteryseparator made from a porous polypropylene film which is one example ofthe polyolefin microporous film of the present invention is illustratedhereinbelow. Incidentally, the kinds of constituent materials are notlimited to those in the following examples.

As an electrolyte diaphragm, the aforesaid porous polypropylene filmwhich is impregnated with a liquid electrolyte or a gelatinizedelectrolyte is employed. As the liquid electrolyte, a nonaqueous solventin which lithium ion salt is dissolved is employed.

Typical examples of the nonaqueous solvent are propylene carbonate,γ-butyrolactone, dimethoxyethane, dioxolane, tetrahydrofuran, dimethylsulfoxide, ethylene carbonate, acetonitrile, sulfolane, low molecularweight liquid polyethylene oxide, and compounds alkyl ether group in theterminal portion thereof. These materials may be used in a mixed form.

Typical examples of lithium ion salt are LiClO₄, LiB(C₆ H₅)₄, LiCF₃ SO₃,LiPF₆. These materials may be used in a mixed form.

The gelatinized electrolyte is obtained by mixing the aforesaid liquidelectrolyte with a polymeric material and gelatinizing the mixture.Typical examples of the polymeric materials are polyethylene oxide,polymethacrylate, a polymer of acryloyl denatured polyethylene oxide,polyvinylidene fluoride, polyvinyl methylether, polyacrylonitrile orcopolymers thereof.

Typical examples of active materials used as the cathode are lithium anda lithium alloy such as an alloy of lithium, aluminum, mercury, zinc,etc.

Typical examples of active material used as the anode are manganesedioxide, molybdenum trioxide, vanadium pentoxide, titanium, niobiumsulfide, chromium oxide, and copper oxide. Carbon black as an electricalconducting material and, if necessary, a binder such aspolytetrafluoroethylene are mixed with such an active material and themixture obtained is formed under pressure to prepare an anode layer.

This type of battery separator is produced from a microporouspolypropylene film, and therefore excels in chemical resistance.Accordingly, the battery separator can be adapted to a wide range ofapplications, such as an alkaline battery and a lithium battery. Inaddition, the battery separator can be cut into various forms inaccordance with the configuration of a battery in which it is to beused. The battery separator impregnated with a liquid electrolyte, agelatinized electrolyte, a solid polymeric electrolyte and the like areinterposed between anode and cathode of the battery, thereby realizingthe function of preventing short-circuiting from occurring between bothelectrodes as well as the function of ionic conduction.

As compared with the conventional porous polypropylene film, the porouspolypropylene film according to the present invention has a highporosity, a uniform pore configuration, an increased number ofthrough-pores, and a uniform pore size. Accordingly, the porouspolypropylene film of the present invention excels in theabove-described functions.

If the porosity of the film is less than 45%, the ionic permeabilityfalls and the internal resistance increases. If the porosity isincreased in the conventional production method, the pore size becomesnonuniform. However, as in the case of the porous polypropylene film ofthe present invention, even if the porosity is increased, the uniformityof the pore size is maintained and an excellent separator function canbe achieved.

A method of producing microporous film in accordance with the presentinvention will be described in detail below.

For the polyolefins useful in producing the micro-porous film of thepresent invention, high-density polyethylene, polypropylene and poly(4-methylpentene-1), for example, can be used. Polypropylene isparticularly preferred, and the kind of polypropylene to be used is notlimited to any particular one. Not only a polymer consisting ofpropylene but a random, block or graft copolymer of propylene and othermonomer or an oligomer may be employed. (In the description of thepresent invention, if the term "polypropylene" is used with no specificdefinition, it is the general term for the above-described polymer andcopolymer.) Materials usable as the monomer or oligomer described abovemay be of any type that can be copolymerized and, for example, ethyleneor an oligomer derived from ethylene may be used.

The melt viscosity [melt flow index (MFI) or melt index (MI)] ofpolyolefin to be employed is not particularly limited to a range withinwhich formation of the film is allowed. As an example, if theformability or productivity of film is taken into account, it ispreferable to use polypropylene of a type whose MFI is 0.5-40 g/10 min.

Polyolefin including additives such as a plasticizer, a colorant, aflame retardant and a filler may be employed in place of the abovepolypropylene.

In the present invention, an unstretched polyolefin film is, prepared byeffecting formation based on a known film producing method. Typicalexamples of usable film producing methods are an inflation film formingmethod and a T die film forming method. The forming conditions of such aforming method can be freely selected in accordance with which of theknown forming methods is used. For example, the film-forming temperaturemay be selected from among temperatures which are not lower than atemperature at which the polyolefin employed can be delivered and whichare not higher than a temperature at which thermal decomposition of thepolyolefin occurs. If polypropylene is used as polyolefin, thefilm-forming temperature is normally 170°-300° C., preferably 190°-270°C.; in the case of high-density polyethylene, normally 150°-300° C.,preferably 160°-270° C.; and in the case of poly(4-methylpentene-1),normally 260°-330° C., preferably 270°-300° C.

Moreover, the elastic recovery of the unstretched polyolefin filmobtained by the above-described formation, for example, that of anunstretched polypropylene film, is not specifically limited. However, inthe case of an unstretched polypropylene film whose elastic recovery iszero % or extremely close thereto, that is, the extent of crystallineorientation of which is extremely small, even if such an unstretchedpolypropylene film is subjected to a stretching step of the presentinvention, it is difficult to obtain microporous film having a distinctpore structure which can be achieved in accordance with the presentinvention. Accordingly, it is desirable to set the forming conditions ofunstretched film while taking account of characteristics such as theporosity of the desired porous polypropylene film and the average poresize of the fine pores.

As described above, the elastic recovery of unstretched polypropylenefilm is not particularly limited, but, for the above-described reasons,it is desirable for the elastic recovery of unstretched polypropylenefilm from a 50% stretched state thereof at a 25° C. with a relativehumidity of 65% to be set to 20% or more. Moreover, if productivitywhich may result from the use of an ordinary forming apparatus is takeninto account, it is particularly preferable that the elastic recoveryrange from 30 to 95%. ##EQU1##

Moreover, on condition that the above requirements and other primaryfactors such as productivity be taken account of, the draft ratio (theratio of the take-up speed of unstretched thermoplastic resin film tothe speed of delivery from a die : the ratio of take-up speed todelivery speed) of unstretched polypropylene film according to thepresent invention will preferably be in the range of 10-6,000 in termsof productivity. In addition, it is necessary to form film whilesufficiently considering the kind of cooling medium, the time anddistance required for delivered resin to make contact with the coolantmedium, the temperature of the resin during this step, and the like,because a value of elastic recovery of unstretched film strongly dependsupon cooling condition during film forming, even if a draft ratio is thesame.

The unstretched polyolefin film thus obtained may be subjected to heattreatment prior to a stretching step. Since the crystallinity of theunstretched polyolefin film can be enhanced by applying the heattreatment before the stretching step, the characteristics of themicroporous film obtained by stretching are further improved. The heattreatment is carried out by using, for example, a method in whichunstretched polyolefin film is heated for 3 seconds or more in airheated to a temperature which is lower than the melting point ofpolyolefin by 15°-17° C.

Unstretched polyolefin film which can be suitably subjected to thestretching step according to the present invention is of a type whosecrystallinity is 50-90%, preferably 60-90%, whose diffraction imagesobtained by small angle X-ray scattering are two-point figures eachhaving an approximately circular form, the half-value width of themaximum degree of diffraction of each of the figures being 15' or less,preferably 12' or less with the long period 120Å or more, preferably150Å or more.

The stretching step according to the present invention is selected fromamong the following methods.

1) The stretching step is effected in a medium selected from the groupconsisting of nitrogen, oxygen, argon, carbon monoxide, methane andethane at a stretching temperature of -70° C. or below and within thelow-temperature range from the solidifying point of the medium to atemperature which is 50° C. higher than the boiling point of the same.Alternatively,

2) The stretching step is effected at a strain rate corresponding to astrain rate of less than 10%/min. in the lowest temperature within atemperature range which is 10°-60° C. lower than the melting point ofthe polyolefin employed, without any pre-stretching step at roomtemperature.

If polypropylene used as polyolefin is to be stretched by method 2), itis preferable that the range of stretching temperatures be 110°-155° C.

First of all, method 1) is described below.

In the extremely low temperature stretching step according to method 1),the aforesaid media may be used independently or in a mixed form.

Examples of stretching temperatures suitable for use of the above mediaare -209° to -146° C. with respect to nitrogen, -218° to -132° C. withrespect to oxygen, -189° to -140° C. with respect to argon, -205° to-141° C. with respect to carbon monoxide, -182° to -111° C. with respectto methane, -183° to -70° C. with respect to ethane. If the stretchingtemperature is higher than -70° C., the percentage of available poreswhich can be formed by stretching, for example, an unstretched film oflow elastic recovery becomes low. In method 1), the temperature rangenot higher than a temperature which is 50° C. higher than the boilingpoint does not mean a temperature range which is lower than atemperature range which is accurately 50° C. higher than the boilingpoint, but means a temperature range not higher than a temperature whichis approximately 50° C. higher than the boiling point.

Under such extremely low temperature conditions, any of the aforesaidmedia assumes a liquid state, a liquid-gas state, or a gas state.However, the above-described stretching step can be effected no matterwhich state the medium assumes.

It is presumed that the above-described stretching according to thepresent invention is brought about as the result of crazing caused bystretching in the aforesaid medium at such extremely low temperatures.The number of crazes generating in this stretching step increases withincreasing strain rate, but the depth of crazes decreases withincreasing strain rate. Namely this fact means that an adequate strainrate must be selected depending upon the thickness of unstretched filmin order to get a film with through-pores. In the ordinary media otherthan the aforesaid media, a polyolefin film assumes a glass state underextremely low temperature conditions and is therefore cut with noextension appearing by stretching. Accordingly, no crazing occurs.

The extremely low temperature stretching of the present invention can beimplemented in a temperature range from the solidifying point of themedium employed to a temperature not higher than a temperature which is50° C. higher than the boiling point. In general, it is advantageous toeffect stretching at temperatures close to the boiling point of the lowtemperature liquid from the viewpoints of production control, as well asin order to keep constant the characteristics of the polyolefinmicroporous film to be produced.

The percentage of stretching in the above-described extremely lowtemperature stretching step is generally in the range of 1 to 200% withrespect to the initial length of the unstretched polyolefin film.However, the suitable percentage of stretching is in the range of 10 to150%. Within the percentage of stretching described above, as thepercentage of stretching increases, the number of pores and the porositytend to increase, respectively. If this tendency is utilized, theaverage pore size or porosity of polyolefin microporous film to beproduced can be adjusted for specific purposes.

The above-described extremely low temperature stretching step may berepeated two times or more until the desired pore structure, the desiredaverage pore size, the desired porosity and the desired mechanicalproperties are obtained.

Unlike the conventional process utilizing a stretching step in thevicinity of room temperature, the present inventive process of makingporous a polyolefin film such as a polypropylene film in theabove-described specific medium by utilizing a stretching step undercooling conditions at extremely low temperatures can also be effectivelyapplied to a polypropylene film whose elastic recovery from a 50% strainat 25° C. is less than 40%. Accordingly, it is possible to produce apolyolefin microporous film such as an excellent porous polypropylenefilm which has uniform pores and high porosity.

The polyolefin film which has been made porous through the extremely lowtemperature stretching step in the aforesaid specific medium is thenpreferably subjected to heat fixation. The primary object of this heatfixation is to thermally fix the film so as to retain the fine poresthus formed. The heat fixation is implemented by using a method ofheating a polyolefin film, which has been made porous while maintainingthe stretched state at extremely low temperatures, for 3 seconds or morein air which is heated to a temperature which is 5°-60° C. lower thanthe melting temperature of the polyolefin employed. In the method ofheating a polyolefin film in heated air for 3 seconds or more, specificexamples of the heating temperatures are: with respect to polypropylene,normally 110°-165° C., preferably 130°-155° C.; with respect tohigh-density polyethylene, normally 70°-125° C., preferably 80°-120° C;and with respect to poly (4-methylpentene-1), normally 150°-210° C.,preferably 160°-200° C. If the heating temperature of a particularmaterial is remarkably higher than the upper limit of the temperaturedescribed above, the formed fine pores may be closed. If the heatingtemperature is remarkably lower than the lower limit, or if the heatingperiod is shorter than 3 seconds, insufficient heat fixation results andthe formed pores may be closed at a later time. Moreover, the film maybe susceptible to heat shrinkage due to temperature changes during use.The above-described extremely low temperature stretching and heatfixation can be repeatedly conducted until the desired average pore sizeand porosity are obtained. In other words, a process comprising thesteps of changing the temperature of the film to room temperature andsubjecting the film to the extremely low temperature stretching (andheat fixation) can be carried out repeatedly. If the extremely lowtemperature stretching is conducted repeatedly, the number of pores tobe formed can be increased and the porosity and the average pore sizecan also be increased.

The polyolefin microporous film, such as a porous polypropylene film,prepared in the above-described manner exhibit good characteristicssince the average pore size is large and the porosity is high. If such apolyolefin microporous film is subjected to a heat stretching step, itscharacteristics will be further improved.

The heat stretching step to be applied to the polyolefin film which hasbeen made porous through at least one cycle of the aforesaid extremelylow temperature stretching step is implemented in the following manner.The primary object of this heat stretching step is to enlarge the poresize of fine pores which have been formed at extremely low temperatures.This heat stretching step is implemented by using a method of stretchinga porous polyolefin film in air heated to a temperature which is 90°-5°C. lower than the melting temperature of the polyolefin employed. Theheat stretching step is implemented by effecting stretching in air whichis heated to, for example, normally 80°-160° C., preferably 110°-155° C.with respect to a polypropylene film; normally 70°-125° C., preferably80°-120° C. with respect to high-density polyethylene; and normally150°-210° C., preferably 160°-200° C. with respect to poly(4-methylpentene-1). If the heating temperature of a particular materialis higher than the upper limit of the aforesaid temperature, the formedfine pores may be closed. If the heating temperature is lower than thelower limit, a sufficient degree of enlargement of the pore size may notbe obtained by stretching.

The percentage of stretching in the above-described heat stretching stepis normally 10-700%, preferably 50- 550% with respect to the length of afilm before the extremely low temperature stretching step (the initiallength). If the percentage of stretching is lower than 10%, the degreeof enlargement of the pore size is insufficient. If it is higher than700%, the film may break off.

This heat stretching step and the aforesaid extremely low temperaturestretching step may be conducted alternately, or the heat stretchingstep may be conducted after one cycle of extremely low temperaturestretching step has been completed.

It is desirable that the film which has been made porous by theaforesaid stretching step be subjected to heat fixation at the intervalbetween this stretching step and the next stretching step. The primaryobject of the heat fixation step is to thermally fix the film so as toretain the pores formed in the heat stretching step.

This heat fixation step is generally implemented by using a method ofheating a polyolefin film which has been made porous, at a temperaturewhich is 5°-60° C. lower than the melting temperature of the polyolefinemployed, in air for 3 seconds or more with its stretched statemaintained. Specific examples of the heating temperature are: withrespect to polypropylene, normally 110°-165° C., preferably 130°-155°C.; with respect to high-density polyethylene, normally 70°-125° C.,preferably 80°-120° C.; and with respect to poly (4-methylpentene-1),normally 150°-210° C., preferably 160°-200° C.

It is desirable that this heat fixation be similarly effected withrespect to a film which has been passed through all the stretchingsteps.

If the heating temperature of a particular material is higher than theupper limit of the aforesaid temperature, the formed pores may beclosed. If the heating temperature is lower than the lower limit, or ifthe heating period is shorter than 3 seconds, insufficient heat fixationresults and the formed pores may be closed at a later time. Moreover,the film may be susceptible to heat shrinkage due to temperature changesduring use.

The stretching method 2) according to the present invention will beexplained below.

The stretching step in this method 2) is conducted at a strain ratecorresponding to a strain rate of less than 10%/min. in the lowesttemperature within a temperature range which is 10°-60° C. lower thanthe melting temperature of polyolefin, preferably a temperature rangewhich is 20°-60° C. lower than the same. For example, the stretchingstep is conducted at the strain rate corresponding to the strain rate ofless than 10%/min. in the lowest temperature within a temperature rangeof normally 110° to 160° C., preferably 110° to 155° C. with respect topolypropylene; normally 70° to 120° C., preferably 70° to 110° C. withrespect to high-density polyethylene; or normally 150° to 205° C.,preferably 150° to 195° C. with respect to poly(4-methylpentene-1).

The strain rate of less than 10%/min. means the upper limit of thepreferable strain rate in the lowest temperature within the temperaturerange, for example, at 110° C. with respect to polypropylene, and doesnot mean the upper limit of the preferable strain rate in a temperaturehigher than the lowest temperature. Namely, a high-molecular material isviscoelastic body and its stretching behavior depends upon thetime-temperature superposition principle. Therefore, the small strainrate in a low temperature is equivalent to the large strain rate in ahigh temperature. This means, for instance, that the strain ratecorresponding to the strain rate of 10%/min. at 110° C. with respect topolypropylene is larger than 10%/min. in a temperature higher than 110°C.

If stretching is effected at a temperature outside of the aforesaidtemperature range, the following problems will be experienced. In thecase of a temperature which is lower than the aforesaid temperaturerange, film having a small pore size can only be obtained. Moreover,film may break off at a portion whose percentage of stretching is small,and film of low porosity can only be obtained.

On the other hand, if the stretching temperature is higher than theaforesaid temperature range, both the film thickness and the film widthperpendicular to the direction of film stretching may be reduced.Alternatively, the polyolefin may melt or partially melt, so that nopore is formed or small-diameter pores are only obtained.

If the strain rate is larger than a strain rate corresponding to astrain rate of less than 10%/min. in the lowest temperature within thetemperature range, film having small-diameter pores only may be obtainedor no pores may be formed.

If the strain rate is less than a strain rate corresponding to a strainrate of less than 10%/min. in the lowest temperature within thetemperature range, as the percentage of stretching is increased, theaverage pore size and the porosity will increase.

The percentage of stretching can be altered in accordance with theaverage pore size corresponding to the desired use of a selectedpolyolefin microporous film, for example, a porous polypropylene film.The percentage of stretching is set to 100-700%, preferably 150-600%with respect to the initial length of the unstretched polyolefin film.If the percentage of stretching exceeds 700%, the film may break off.

The polyolefin film which has been made porous through the aforesaidstretching step is then preferably subjected to heat treatment. Theprimary object of this heat treatment is to thermally fix the film so asto retain the fine pores thus formed. This heat treatment is implementedby utilizing a method of heating the porous polyolefin film in air at atemperature 5°-60° C. lower than the melting temperature of thepolyolefin employed for 3 seconds or more while maintaining thestretched state thereof. Specific examples of the heating temperatureare: with respect to polypropylene, normally 110°-165° C., preferably130°-155° C.; with respect to high-density polyethylene, normally70°-125° C., preferably 80°-120° C.; and with respect to poly(4-methylpentene-1), normally 150°-210° C., preferably 160°-200° C.

If the heating temperature of a particular material is remarkably higherthan the upper limit of the temperature described above, the formed finepores may be closed. If the heating temperature is remarkably lower thanthe lower limit, or if the heating period is shorter than 3 seconds,insufficient heat fixation results and the formed pores may be closed ata later time. Moreover, the film may be susceptible to heat shrinkagedue to temperature changes during use.

The polyolefin microporous film which was prepared by stretching, byeach of methods 1) and 2), the polyolefin film having the aforesaidcrystallinity, the small angle X-ray scattering pattern, the half-valuewidth of the degree of diffraction and a long period were observed witha scanning electron microscope. It was found that the polyolefinmicroporous film was made up of: groups of unstretched planar flatportions which ran at approximately constant intervals in the directionperpendicular to the direction in which the film was stretched and whichwere formed approximately parallel to a cross section perpendicular tothe direction in which the film was stretched; and groups of relativelyslender fibrils which ran in the gap between adjacent planar flatportions at approximately constant intervals and approximately inparallel with the direction in which the film was stretched. It wasfurther found that a multiplicity of fine pores were formed, each ofwhich had an approximately uniform configuration in which the intervalsbetween the slender fibrils connected adjacent planar flat portionsextended approximately two-dimensionally. In addition, a majority of thefine pores were through-pores and had approximately uniform pore sizes.The film having such a distinct structure is of course suitable for useas a separating membrane for water filtration, plasma separation or thelike, and is extremely suitable for use as a battery separator as well.

EXAMPLES

The present invention will be explained more specifically with referenceto examples and comparative examples, but the following examples are notintended to limit the scope of the present invention.

Incidentally, the permeability illustrated in each of the followingexamples and comparative examples was obtained through the steps ofimparting hydrophilic properties to the porous film in accordance with amethod described in ASTM-F317, setting the hydrophilic porous film in apredetermined holder, applying pressure to one side of the film in thedirection of the thickness thereof, and measuring the quantity of waterpassed per unit time through the film from the pressed side to the otherside.

Examples 1 to 4 and comparative examples 1 and 2 relate to themicroporous film according to the present invention, and examples 5 to10 and comparative examples 3 and 4 relate to a battery separator usinga porous polypropylene film of the present invention.

EXAMPLE 1

Polypropylene (brandname: UBE-PP-F109K made by Ube Industries, Ltd.MFI=9 g/10min.,) was formed into an inflation film by using an inflationmolder provided with a die having an outer diameter of 150 mm under theconditions of a delivery temperature of 190° C. and a take-up speed of30 m/min. The polypropylene film thus obtained was heated for 30 minutesin a heated air oven held at 145° C., thereby preparing an unstretchedpolypropylene film of the type whose crystallinity was 70% and whosediffraction images obtained by small angle X-ray scattering weretwo-point figures each having an approximately circular form, thehalf-value width of the degree of diffraction being 10' with the longperiod thereof 211 Å.

This unstretched film was stretched in liquid nitrogen (-196° C.) by 20%with respect to the initial length. Subsequently, the film was subjectedto heat fixation for 2 minutes in a heated air oven held at 145° C. withthe stretched state maintained.

After this film was subjected to hot stretching by 300% in an atmosphereof air at 130° C., it was further subjected to heat fixation for 30minutes in a heated air oven held at 145° C., thereby producing thepolypropylene microporous film.

When the average pore size (D_(A)) and the maximum pore size (D_(M)) ofthe polypropylene microporous film thus obtained were measured by ahalf-dry method employing ethanol which is based on the method in ASTMF316-30, it was found that the average pore size and the D_(A) /D_(M)ratio were 0.13 μm and 0.85, respectively. When the porosity wasmeasured by a method of charging mercury by pressure by means ofPorosimetro Series 1500 manufactured by Carloerba Company, Italy, it wasfound to be 71.0%. When the tensile elastic modulus in the stretchingdirection was measured in accordance with the method described in ASTMD882, it was found to be 7,700 kg/cm². The water filtration rate of thefilm to which hydrophilic properties were imparted with ethanol was 30.0l/m² ·min.·kg/cm². When the surface and the cross section of thispolypropylene microporous film were observed with a scanning electronmicroscope (X-605 manufactured by Hitachi, Ltd.), it was found that thepolypropylene microporous film comprised groups of planar flat portionswhich were formed approximately perpendicularly to the direction inwhich the film was stretched and groups of fine fibrils which wereformed approximately parallel to each other in the direction in whichthe film was stretched, these two groups forming a multiplicity of finethrough-pores which extended approximately two-dimensionally. Theelectron micrographs of the surface and the cross section of thethus-obtained polypropylene microporous film are shown in FIG. 1 (amagnification of 12,400×) and FIG. 2 (a magnification of 4,500×),respectively. Incidentally, the half-dry method, Porosimetro Series1500, the method described in ASTM D882 and the scanning electronmicroscope (X-605 ) are similarly employed in each of the followingexamples.

COMPARATIVE EXAMPLE 1

The same polypropylene as that used in Example 1 was employed to preparean unstretched polypropylene film of the type whose crystallinity was40% and whose diffraction images obtained by small angle X-rayscattering were two-point figures each having an elliptical form, thehalf-value width of the degree of diffraction in the short axis of eachof the ellipses being 20' with the long period thereof 165 Å.

This unstretched film was subjected to stretching and heat fixationsteps similar those used in Example 1, thereby producing thepolypropylene microporous film.

When the surface and the cross section of this polypropylene microporousfilm were observed with the scanning electron microscope, neither groupsof planar flat portions which were formed approximately perpendicularlyto the direction in which the film was stretched nor a multiplicity offine through-pores of uniform size and configuration were observed. Thepores appearing at the surface and the cross section were not uniform insize or configuration, and portions which formed the walls between thefibrils were present in a meandering manner to form closed circuits ofvarious sizes. No through-pores which extended two-dimensionally wereobserved.

EXAMPLE 2

The polypropylene film formed under the same conditions as those used inExample 1 was subjected to heat treatment for 20 minutes in a heated airoven held at 140° C. to prepare an unstretched polypropylene film of thetype whose crystallinity was 68% and whose diffraction images obtainedby small angle X-ray scattering were two-point figures each having anapproximately circular form, the half-value width of the degree ofdiffraction being 11' with the long period thereof 200 Å.

This unstretched film was stretched by 300% with respect to the initiallength at a temperature of 145° C. and a strain rate of 8.33%/min, andwas then subjected to heat fixation for 10 minutes in a heated air ovenheld at 145° C. with the stretched state maintained, thereby producingthe polypropylene microporous film.

When the surface and the cross section of this polypropylene microporousfilm were observed with the scanning electron microscope, a structuresimilar to that of the film obtained in Example 1 was observed.

COMPARATIVE EXAMPLE 2

The same polypropylene as that used in Example 1 was employed to producea polypropylene microporous film by stretching under the same conditionsexcept that stretching in air (25° C.) was substituted for stretching inliquid nitrogen.

Although the unstretched polypropylene film was identical to that usedin Example 1, the average pore size, the porosity, the D_(A) /D_(m)ratio, the elastic modulus and the water filtration rate of the obtainedmicroporous film exhibited the respective values lower than those of theporous polypropylene film obtained in Example 1. Moreover, when thesurface and the cross section of this polypropylene microporous filmwere observed with the scanning electron microscope, the number of poreswhich extended from one to the other surface was small compared to thefilm obtained in Example 1.

EXAMPLE 3

The same polypropylene film as that used in Example 1 was processed in amanner similar to that used in Example 1 except that stretching wasconducted in argon held at -180° C.

When the surface and the cross section of this polypropylene microporousfilm were observed with the scanning electron microscope, a structuresimilar to that of the film obtained in Example 1 was observed.

EXAMPLE 4

The same polypropylene film as that used in Example 1 was stretched by20% with respect to the initial length in liquid nitrogen (-196° C.),and was then subjected to heat fixation for 2 minutes in a heated airoven held at 145° C. with the stretched state maintained, and thisstretching and the following heat fixation were repeated five times.

When the surface and the cross section of this polypropylene microporousfilm were observed with the scanning electron microscope, a structuresimilar to that of the film obtained in Example 1 was observed.

The characteristics of the respective films obtained in examples 2 to 4and comparative examples 1 and 2 are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                 Comparative                                                                          Comparative                                          Example 2                                                                           Example 3                                                                           Example 4                                                                           example 1                                                                            example 2                                 __________________________________________________________________________    Film       25    26    25    25     27                                        thickness (μm)                                                             Average pore                                                                             0.20  0.11  0.30  0.03   0.07                                      diameter (μm)                                                              D.sub.A /D.sub.M                                                                         0.83  0.92  0.71  0.43   0.40                                      Porosity (%)                                                                             73.1  69    72    21     28                                        Elastic    7300  7400  6900  3800   4600                                      modulus (kg/cm.sup.2)                                                         Water filtration                                                                         43.0  19.1  78.5  1.1    2.3                                       rate                                                                          (l/m.sup.2 · min · kg/cm.sup.2 )                            __________________________________________________________________________

EXAMPLE 5

An unstretched polypropylene film was formed by means of an inflationmolder provided with an inflation forming die having a diameter of 50 mmand a slit gap of 0.7 mm. Forming was effected by a method comprisingthe steps of delivering polypropylene at a resin delivery temperature of210° C. while blowing air at room temperature into a valve at a blowratio of 0.7, then blowing air onto the outer wall surface of thedelivered film at a location 5 cm above the die for cooling purposes,and then taking up it with a nip roll at a take-up speed of 36 m/min. ata location 1.8 m above the die, thereby producing the desiredunstretched polypropylene film.

The polypropylene employed were as follows.

Polypropylene : Ube-PP-Y101J (manufactured by Ube Industries, Ltd.)

MFI=1 g/10min.

The thickness of the thus-obtained unstretched film was 10 μm. Theelastic recovery of this film from a 50% stretched state was 80%.

The obtained film was subjected to heat treatment for 10 minutes at 145°C. with no tension applied thereto. The elastic recovery of the obtainedfilm was improved to 87%.

The crystallinity of the obtained film was 68% and the diffractionimages obtained by small angle X-ray scattering were two-point figureseach having an approximately circular form, the half-value width of thedegree of diffraction being 11' with the long period thereof 135 Å.

This unstretched film was stretched in liquid nitrogen (-196° C.) by 10%with respect to the initial length. Subsequently, the film was subjectedto heat fixation for 10 minutes in a heated air oven held at 145° C.with the stretched state maintained. After the heat fixation, the filmwas further stretched by 180% with respect to the length provided by thestretching in the liquid nitrogen. Subsequently, the film was subjectedto heat fixation for 10 minutes in a heated air oven held at 145° C.with the stretched state maintained, thereby producing the polypropylenemicroporous film.

When the average pore size of the polypropylene microporous film thusobtained was measured by a half-dry method employing ethanol which wasbased on the method described in ASTM F316-80, it was found to be 0.13μm. When the porosity was measured by a method of charging mercury bypressure (by means of Porosimetro Series 1500 manufactured by CarloerbaCompany, Italy), it was found to be 54.60%. The water filtration ratewas 23.18 l/m² ·min.·kg/cm².

When the surface and the cross section of this porous polypropylene filmwere observed with a scanning electron microscope, it was found that thepolyolefin was made up of groups of unstretched planar flat portionswhich ran at approximately constant intervals in the directionperpendicular to the direction in which the film was stretched and whichwere formed approximately parallel to a cross section perpendicular tothe direction in which the film was stretched, and groups of relativelyslender fibrils which ran in the gap between adjacent planar flatportions at approximately constant intervals and approximately inparallel with the direction in which the film was stretched. It wasfurther found that a multiplicity of fine pores were formed, each ofwhich had an approximately uniform configuration in which the intervalsbetween the slender fibrils connecting adjacent planar flat portionsextended approximately two-dimensionally. In addition, when the crosssection of the film was observed, a majority of the formed fine poreswere pores which extended from one surface to the other surface.

The results of measurement of the electrical conductivity of this filmare shown in Table 2.

Incidentally, the above half-dry method was also employed in thefollowing examples.

COMPARATIVE EXAMPLE 3

The same polypropylene film as that used in Example 5 was employed toproduce a polypropylene microporous film by stretching under the sameconditions except that stretching in air (25° C.) was substituted forstretching in liquid nitrogen.

When the average pore size of the polypropylene finely porous film thusobtained was measured by a half-dry method, it was found that theaverage pore size was 0.07 μm. When the porosity was measured by meansof a method of charging mercury under pressure, it was found to be41.50%. The water filtration rate was 6.66 l/m² ·min.·kg/cm².

Although the unstretched polypropylene film was identical to that usedin Example 5, the average pore size, the porosity, and the waterfiltration rate of the obtained porous film exhibited values lower thanthose of the porous polypropylene film obtained in Example 5. Moreover,when the surface and the cross section of the porous polypropylene filmobtained were observed with a scanning electron microscope, the numberof pores which extended from one to the other surface was small comparedto the film obtained in Example 5. The results of measurement of theelectrical conductivity of this film is shown in Table 2.

EXAMPLE 6

An unstretched film was formed by using an apparatus similar to thatused in Example 5. The polypropylene employed and the forming conditionsare as follows.

Polypropylene : UBE-PP-J130G (manufactured by Ube Industries, Ltd.)

MFI=30 g/10 min.

Resin delivery temperature : 170° C.

Take-up speed : 36 m/min.

Blow ratio : 0.70

The thickness of the thus-obtained unstretched film was 20 μm. Theelastic recovery of this film from a 50% stretched state was 30%.

The obtained film was subjected to heat treatment for 10 minutes at 145°C. with no tension applied thereto. The elastic recovery of the obtainedfilm was improved to 72%.

This unstretched film was subjected to stretching in liquid nitrogen andheat fixation in a heated air oven in a manner similar to that carriedout in Example 5.

This film was stretched by 330% with respect to the length provided bystretching in the liquid nitrogen, in a heated air oven held at 145° C.,and was then subjected to heat fixation for 10 minutes in a heated airoven held at 145° C. with the stretched state maintained, therebyproducing a porous polypropylene film.

When the average pore size of the porous polypropylene film thusobtained was measured by the half-dry method, it was found to be 0.70μm. When the porosity was measured with a mercury porosimeter, it wasfound to be 65.30%. The water filtration rate was 59.00 l/m²·min.·kg/cm².

When the surface and the cross section of the porous polypropylene filmobtained were observed with the scanning electron microscope, the poreswere substantially uniformly distributed over the surface of the filmand the pore size was substantially uniform over the entire film. Whenthe cross section of this film was observed with the scanning electronmicroscope, a majority of the formed pores were pores which extendedfrom one to the other surface. The results of measurement of theelectrical conductivity of this film are shown in Table 2.

EXAMPLE 7

Polypropylene (brandname: UBE-PP-F109K made by Ube Industries, Ltd.MFI=9 g/10 min.,) was formed into an unstretched polypropylene film bymeans of an inflation molder provided with an inflation forming diehaving a diameter of 50 mm and slit gap of 0.7 mm. Forming was effectedby a method comprising the steps of delivering polypropylene at a resindelivery temperature of 200° C. while blowing air into a valve at a blowratio of 0.7, then blowing air at room temperature onto the outer wallsurface of the delivered film at a location 5 cm above the die forcooling purposes, and then taking up it with a nip roll at a take-upspeed of 36 m/min. at a location 1.8 m above the die, thereby producingthe desired unstretched polypropylene film.

The thickness of the thus-obtained unstretched film was 20 μm. Theelastic recovery of this film from a 50% stretched state was 38%.

The obtained film was subjected to heat treatment for 10 minutes at 145°C. with no tension applied thereto. The elastic recovery of the obtainedfilm was improved to 85%.

This unstretched film was stretched by 300% with respect to the initiallength at a temperature of 145° C. at a strain rate of 8.33%/min.Subsequently, the film was subjected to heat fixation for 10 minutes ina heated air oven held at 145° C. with the stretched state maintained,thereby producing the polypropylene microporous film.

When the average pore size and the porosity of this film were measuredby the same method as that used in Example 5, they were found to be 0.4μm and 67%, respectively. The results of measurement of the electricalconductivity of this film are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Measurement of Electrical Conductivity                                                Porosity (%)                                                                           Electrical conductivity (S/cm)                               ______________________________________                                        Example 5 54.6       5.10 × 10.sup.-3                                   Example 6 65.3       5.16 × 10.sup.-3                                   Example 7 67.0       5.21 × 10.sup.-3                                   Comparative                                                                             41.5       2.48 × 10.sup.-3                                   example 3                                                                     ______________________________________                                    

Method of measuring electrical conductivity

A porous polypropylene film was impregnated with a liquid electrolyte(an isovolumetric mixture solution of propylene carbonate anddimethoxyethane, in which a 1-mol concentration of lithium perchloratewas dissolved), and the electrical conductivity was measured with aconductivity meter.

EXAMPLE 8

A metallic lithium disk having a diameter of 16 mm and a thickness of0.75 mm was prepared as the cathode. For the anode, 68 mg of manganesedioxide, 8.5 mg of acetylene black and, as a binder, 8.5 mg of Teflonpowder (Luflon L-5 manufactured by Daikin Kogyo Co., Ltd.) were mixed,and the mixture was formed under a pressure of 7.5 ton/cm².Subsequently, the thus-formed mixture was subjected to heat treatment at300° C., thereby preparing an anode layer having a diameter of 13 mm anda thickness of 1.3 mm.

As a porous polypropylene film, the film produced in Example 5 wasemployed.

This porous polypropylene film was impregnated with a liquid electrolyte(an isovolumetric mixture solution of propylene carbonate anddimethoxyethane, in which a 1-mol concentration of lithium perchloratewas dissolved), and the film thus obtained was sandwiched between theanode and cathode to prepare a battery.

As a battery performance test, a 2.8 k Ω constant-resistance dischargetest was performed to obtain a battery capacity of 16 mAh with a2.0-volt cut.

COMPARATIVE EXAMPLE 4

The film produced in Comparative Example 3 was employed to prepare alithium battery in a manner similar to that used in Example 8. When abattery performance test similar to that in Example 8 was conducted, thedischarge capacity was found to be 52 mAh.

EXAMPLE 9

The film produced in Example 6 was employed to prepare a lithium batteryin a manner similar to that used in Example 8. When a batteryperformance test similar to that in Example 8 was conducted, thedischarge capacity was found to be 16 mAh.

EXAMPLE 10

The film produced in Example 7 was employed to prepare a lithium batteryin a manner similar to that used in Example 8. When a batteryperformance test similar to that in Example 8 was conducted, thedischarge capacity was found to be 17 mAh.

The microporous film of the present invention comprises through-pores ofuniform size and configuration which are formed approximatelyperpendicularly to the direction of film stretching and, in addition,the porosity is high. Accordingly, the film can yield both satisfactorywater filtration rate and selective separating capabilities.Accordingly, it is possible to provide a microfilter having excellentfiltration performance which can be adapted to various applications notonly in the field of water treatment, air purification or the like, butin the field of plasma separation, particularly such as separators foruse in donation plasmapheresis. Moreover, the above-describedcharacteristics of the porous film of the present invention make itpossible to extremely suitably adapt the film to applications in abattery separator.

For example, by using the porous film of the present invention, it ispossible to provide a battery separator in which a majority of pores arethrough-pores and have approximately uniform pore sizes and which excelsin characteristics such as ionic conductivity by virtue of the presenceof fine pores of a unique form which yields a porosity of 45% or more.Accordingly, by using this battery separator, it is possible to providea lithium battery having excellent battery performance.

What is claimed is:
 1. A method of producing a polyolefin microporousfilm having a multiplicity of fine through-pores by a stretchingprocess, comprising the steps of:preparing an unstretched polyolefinfilm whose crystallinity is 50-90%, whose diffraction images obtained bysmall angle X-ray scattering are two-point figures each having anapproximately circular form, the magnitude of a diffraction angle beingnot greater than 15' at the degree of diffraction which is equivalent to1/2 (half-value width) of the maximum degree of diffraction of each ofsaid figures, the long period being not smaller than 120Å; andstretching said unstretched film at a strain rate corresponding to astrain rate of less than 10%/min. in the lowest temperature within atemperature range which is 10°-60° C. lower than the melting point of apolyolefin employed, without any pre-stretching step at roomtemperature.
 2. A method of producing a polypropylene microporous filmhaving a multiplicity of fine through-pores obtained by stretching apolypropylene film, comprising the steps of:preparing an unstretchedpolyolefin film whose crystallinity is 50-90%, whose diffraction imagesobtained by small angle X-ray scattering are two-point figures eachhaving an approximately circular form, the magnitude of a diffractionangle being not greater than 15' at the degree of diffraction which isequivalent to 1/2 (half-value width) of the maximum degree ofdiffraction of each of said figures, the long period being not smallerthan 120Å; and stretching said unstretched film at a strain ratecorresponding to a strain rate of less than 10%/min. in the lowesttemperature within the high-temperature range of 110°-155° C., withoutany pre-stretching step at room temperature.
 3. A method of producing abattery separator made from a polypropylene microporous film having amultiplicity of fine through-pores obtained by stretching apolypropylene film, comprising the steps of:preparing an unstretchedpolypropylene film; and stretching said unstretched polypropylene filmat a strain rate corresponding to a strain of less than 10%/min. in thelowest temperature within a high-temperature range of 110° to 155° C.,without any pre-stretching step at room temperature.